Apparatus for determining the dynamic forces on a drill string during drilling operations

ABSTRACT

The Apparatus acting as a counterforce presented for measuring, indicating and recording the weight of the drill string, the torque applied to the drill string and the bending force at the top drive during drilling operations where a plurality of strain gauges for measuring the force of tension, torque and bending with indexing are suitably mounted on the counterforce to optimize the accuracy and limit the cross sensitivity between the forces. The counterforce is installed at the top of the drill string contained within the top drive assembly. Polynomial regression analysis provides the coefficients for digital signal conversion to engineering units. The strain gauges are connected to a circuit board that facilitates external communication for calibration purposes and the sending and receiving of data. The counterforce requires no physical adjustment after manufacture. The circuits contained within the counterforce are battery driven. The apparatus is certified for use in hazardous atmospheres.

The present invention relates to an apparatus for determining the dynamic forces on a drill string during drilling operations including tensile and compressive forces due to axial loading, torsional stress due to the torque applied by the prime mover and the radial stress due to bending. In particular the invention relates to the measurement of the axial weight of the drill string, the torque applied to the drill string at the top drive assembly, the bending forces at the top of the drill string and the velocity of rotation during drilling operations. In addition the invention correlates to the radial position of the measured values by use of an index pulse.

It is an established fact that the velocity of rotation, the axial load, torque and bending applied to a drill string during drilling operations are the parameters that directly effect the successful drilling operation.

As a preliminary to describing the embodiments of the current invention previous attempts at improving measurements of forces on the drill string are described as they are helpful in explaining those embodiments. The axial load applied to the drill bit is also known as “weight-on-bit” or “WOB”. The drill bit has weight applied to it by suspending a succession of drill collars and drill pipes screwed together to form a drill string. The total length of the drill string is connected to the bottom end of the top drive assembly from which the drill string is suspended in tension. The amount of “WOB” can be varied by adjusting the surface drill string weight.

A torque is applied to the drill bit (“torque-on-bit” or “TOB”) through the rotation of the drill string by a top drive mechanism that consists of an electric prime mover coupled to a gear box which is directly coupled via valve subs to the drill string.

Early measurements of drill string weight was made by a hydraulic sensor and gauge mounted in the hoist line at the dead man anchor usually positioned at the base of the derrick. The indicated weight increases as the hydraulic pressure increases due to increased tension on the wire caused by an increase in weight of the drill string. The measurement by its nature also included the weight of the traveling blocks and the cable. There were considerable mechanical losses in the system and errors could be compounded by the large number of falls of line in the traveling block. In order to know the actual weight of the drill string, it is necessary to offset the weight attributed to the traveling blocks, the wire and any other items not part of the drill string. Whilst the total weight of the drill string is of interest to the drilling operator, the actual weight applied to the drill bit at the bottom of the hole is of even greater interest. Most rigs have indicators with two integral dials, the first giving the weight of the drill string after subtraction of the traveling blocks and other appendages and the second scale which can also be set to read zero weight with the drill string hanging free just short of the bottom hole. The dial works backwards so as the bit is engaged with the hole, the reduced or differential weight of the bit is indicated on a scale of increased resolution. The drilling operator is thus able to read the actual weight on the bit within the limitations of the accuracy of the system.

So early attempts at estimating WOB were obtained by monitoring the tension of the hoist line of the draworks (at the bitter end) which raises and lowers the top drive and hence the drill string. The total drill string weight is a product of the single line tension and the number of falls of line applied in the draworks. The weight of the drill string lightens when the bit is on bottom. The WOB is then obtained by subtracting the two values. This method despite the inherent inaccuracy was adequate for vertical wells drilled in shallow water.

As the water depth increased and bore holes deviated from the vertical, alternative methods were employed such as measuring the suspended weight of the top drive and the drill string assembly at the crown block. Whilst more accurate than previous attempts where the difference between the suspended weight and the non-suspended weight (i.e. the “WOB”) was a low value, the lag in responsiveness and accuracy and repeatability although an improvement were still unacceptable.

Torque on the drill string has in the past been calculated by measuring the value of the motor amperes driving the rotary table, the actual value of torque being a mathematical computation taking into account the mechanical advantage of the mechanical system of the rotary table or the top drive assembly.

All further attempts at measurement have all been down-hole, where measurements have been made as close to the drill bit as practical. Several attempts have been made to improve the accuracy including U.S. Pat. No. 3,968,477, to Patton et al., provided a hollow mandrel with a thin section where strain gauges are attached and an outer sleeve to constrain bending. This creates a poor compromise between strength and sensitivity, which then had to be resolved mathematically.

Improvements in drilling methods were achieved with the introduction of the top drive. The top drive rotates the drill string end bit without the use of a kelly and rotary table. The top drive is operated from a control console on the rig floor. The top drive consists of one or more motors connected to the drill string via a gearbox. The top drive is suspended from the traveling block which allows it to travel up and down the derrick. An improvement of the dead man anchor method of measuring the weight on the drill string has been achieved by the application of load sensing devices placed in the crown block. These load sensing devices measure the total weight of the traveling block, the top drive and the drill string, all supported by the crown block assembly. Similarly, an improvement in torque value is possible due to the more direct correlation between motor amperes and actual torque on the drill string.

Having improved measurement of forces on the drill string at the surface attempts have been made to improve the method of measurement of the forces on the drill string using the methodology of bottom-hole devices where the measuring apparatus is installed close to the drill bit. Inherent disadvantages are common to this method of measurement videlicet the inability of the systems to distinguish between values directly related to the applied forces and errors caused by down-hole pressure differential and temperature fluctuations. There are major disadvantages to this type of apparatus, especially in a deepwater situation, in that as the apparatus is at the bottom of the drill string the apparatus is subject to bending forces, vibration and high temperatures. A further disadvantage is that a failure of the device results in the whole of the drill string being tripped (taken out of the hole) to allow a replacement to be fitted. Another criticism is that the forces measured down hole are not representative of the actual forces the drill string experiences.

Embodiments of the present invention are now described and illustrated by example and with reference to the accompanying drawings.

FIG. 1 is a side elevation view of the drill floor and derrick. The positional elements of the apparatus as installed on a drilling rig are illustrated by numbers 1-8.

FIG. 2 is a cross-sectional view through the counter-force. The elements of the preferred embodiment of the invention are illustrated by numbers 20-32.

FIG. 3 is a side elevation view of the drill floor and derrick illustrating an alternative embodiment where the apparatus, 10, is mounted below the topdrive assembly.

FIG. 4 illustrates an array of strain gauges designed to measure the strain of the apparatus in tension

FIG. 5 illustrates an array of strain gauges designed to measure torsional stress in shear of the apparatus under torque.

FIG. 6 illustrates an array of strain gauges designed to measure bending of the apparatus.

The present invention illustrated in FIG. 1 provides an apparatus for measuring tension, torque and bending of the drill string, installed as the lower shaft assembly 1, in the top drive 3, directly above and connected to the drill string 2. The top drive is supported by the draw works 4, which allows the top drive to travel up and down the derrick 5. The invention provides an arrangement of strain gauges that allows measurement of tension, torque and bending within the same device.

In a further embodiment the measurement of the velocity of rotation of the apparatus can be achieved by producing an index pulse once each revolution, the interval between the pulses being timed. The index pulse can also be correlated directly to the tension, torque and bending signals.

The apparatus typically will have suitable threads 6, and 7, at each extremity to allow it to be connected into the drill string.

In yet a further embodiment dual well control valves 29 and 30, both manual and remotely actuated, can be incorporated into the apparatus.

The apparatus thus allows direct measurement of the dynamic forces on the drill string and their conversion to engineering units, videlicet the weight of the drill string, the weight on the bit, the torque on and bending of the drill string hung directly below the apparatus.

In accordance with the above, the present invention provides an apparatus, illustrated in FIG. 2, comprising a suitable thick wall cylinder 1, made from a metal suitable for supporting the weight of the drill string with stepped diameters machined to allow accurate placement of a plurality of strain gauges 22, 23 and 24, for tension, torque and bending measurement. The cylinder acts as a counterforce to the tension, torque and bending forces so applied. The stepped diameters of the cylinder also accommodates the protective outer cover 21. At least one digital circuit board 25, attached to the counterforce is retained in the sealed enclosure 21 to protect the circuit board, the radio telemetry and the transducer bearing portion of the counterforce. The circuit board 25, includes circuits and software for producing numerical values of digital weight, torque, bending and velocity of rotation in engineering units, as well as means for applying stored digital correction factors to the weight, torque and bending readings. Means such as radio modem telemetry 26, provides a signal path through the enclosure to and from the circuit board 25, for external communication. Battery power 27, is the motive force for the circuits embodied in the counterforce and the batteries are so arranged to be easily replaced when necessary. The counterforce requires no physical adjustment within the enclosure after manufacture and can be controlled and corrected using the signal path through the enclosure. The apparatus typically will have suitable threads 6, and 7, at each extremity to allow it to be connected into the drill string. Well control valves 29 and 30, manually and remotely actuated, can be incorporated into the apparatus.

An array of strain gauges designed as a transducer to measure the tensile and compressive forces of the apparatus are illustrated in FIG. 4. In this embodiment eight strain gauges are employed in a full wave bridge to ensure accuracy.

An array of strain gauges designed as a transducer to measure the torsional stress in shear of the apparatus under torque are illustrated in FIG. 5. In this embodiment eight strain gauges are employed in a full wave bridge to ensure accuracy.

An array of strain gauges designed as a transducer to measure radial bending of the apparatus is illustrated in FIG. 6. As the signal of maximum bending is in one plane only, the signal output will produce a cosine curve during rotation. A reed switch, 28, mounted in the apparatus, when switched by the application of an external magnet produces an index pulse such that the pulse occurs at the same radial position as the plane of the maximum bending force. This allows the precise radial position of the bending force when rotating to be determined. It follows that the value of that force can be mathematically determined once the radial position is known.

The output values of weight, torque and bending are transmitted by telemetry means to a receiver which will display the data so received, log and retain the values in memory with date and time stamping. Force values of tension, torque and bending against signal output, and cross sensitivity between the measured values of the said forces are empirically determined. These values have a large dynamic range where typically the lower and higher values tend to have a larger and nonlinear deviation from the norm. A polynomial solution is provided for this type of data analysis for determination of correction factors.

The said correction factors can be accurately defined by a mathematical formula which can be of the form of an N^(th) order polynomial on x, where N is a positive integer representing the greatest order of x in the polynomial.

The general form of this polynomial is:

Z=a ₀ +a ₁ x+a ₂x² +a ₃ x ³ + . . . +a _(N) x ^(N)

Where a₀, a₁, a₂, . . . , a_(N) are the polynomial coefficients.

Where Z represents values in engineering units and x represents the raw measured values

In order to fit or model the system on such, polynomial sets of coefficients are required. These coefficient sets are determined mathematically from a collection of characteristic data points gathered empirically during a calibration procedure. Computational means can be used to determine the coefficients from the data points, using for example a least squares fit, where a series of calculations is carried out to minimise the sum of the squares of the deviations of the predicted points from the data points. The resultant coefficients from each order are easily tested for best fit and the resultant graphed to confirm. Typically the 5^(th) order polynomial will allow a sufficiently accurate approximation of the conditioned values over the complete range of raw values.

A further aspect of the invention allows measurement of an index pulse which allows time correlation of the measured forces and calculation of the velocity of rotation of the apparatus. In this embodiment a reed switch, 28, is used to provide an index pulse once each revolution.

In another aspect of the invention two mud control valves, 29 and 30 in FIG. 2, one manually operated and one remotely operated, can be incorporated into the apparatus.

In yet another aspect of this embodiment the apparatus so described above shall be suitable for use in hazardous atmospheres.

In another embodiment, FIG. 3, the apparatus could be mounted below the top drive.

While preferred and alternative embodiments have been illustrated and described, the scope of the invention is not necessarily limited to such descriptions and illustrations. 

1. The present invention relates to the measurement of the dynamic forces on a drill string during the drilling of a wellbore by application of an apparatus that is mounted at the top of the drill string with means for computing weight, torque, bending, indexing and velocity of rotation of a rotating drill string, comprising a counterforce, transducer means mounted directly on the surface of the said counterforce, circuit means associated with said transducer means, the circuit means associated with the transducers means generates digital signals representative of the axial, torsional and bending forces applied to the said counterforce, the velocity of rotation with an index marker, the said circuit means to be responsive to external communication, means for storing digital correction representations, circuit means responsive to external commands for control, circuit means for storing unique addresses, circuit means for transmitting said digital representations, the transducer and circuit means sealed in an enclosure with means for external communication with said circuit means.
 2. An apparatus as claimed in claim 1 which forms part of the bottom shaft assembly of a top drive, the said counterforce manufactured from a high strength material with suitable end connections compliant with drilling codes of practice and a machined area to house the transducer and circuit means with associated batteries, the whole protected by a machined cover.
 3. An apparatus as claimed in claim 1 where the said transducer means consist of a plurality of strain gauges arranged such to minimise transmigration of signals between said transducers.
 4. An apparatus as claimed in claim 1 where the said circuit means includes a telemetry device as the communicating means.
 5. An apparatus as claimed in claim 1 where mud control valves both manual and remotely operated be included in the body of the apparatus.
 6. An apparatus in accordance with the above claims including computing means and transmitting means; the computing means computing weight of a drill string, torque of a drill string and bending force; the transmitting means transmitting the computed data of weight, torque and bending via radio telemetry and a receptor with radio telemetry means for receiving said data processor means for display of said data computer means for receiving and storing the said data from the electronic load cell and torque transducer.
 7. A transducer in accordance with the above claims measuring tensile and compressive forces, wherein an output signal representative to drill string weight be available for control means.
 8. A transducer in accordance with the above claims measuring torsional stress in shear wherein an output signal representative to the torque applied to the drill string be available for control means.
 9. A transducer in accordance with the above claims measuring bending wherein an output signal representative to the radial bending force on the drill string torque be available for control means.
 10. An index pulse radially linked to the position of the tension, torque and bending signals
 11. Transducers in accordance with claims 7, 8, 9 and 10, wherein an output signal be available in an intranet protocol for remote monitoring and or control.
 12. An enclosure, housing the transducer and circuit means with means for external communication with said circuit means, be certified for use in hazardous atmospheres. 